1. Field of the Invention
This invention, in one aspect thereof, relates to a film which can be effectively perforated by the use of the nature of electromagnetic waves as energy sources generated as by the flash irradiation for a very brief period (such as, for example, 1/1000 second) using a halogen lamp, a xenon lamp, a krypton lamp, or a flashbulb, the infrared irradiation, or the pulse irradiation of laser beam, particularly in the low energy zone. The invention, in another more desirable aspect, relates to a stretched film for a highly heat-sensitive stencil, which is effectively perforated by direct or indirect contact with a low energy source, i.e. a so-called thermal head composed of a multiplicity of fine heating elements and to a stencil sheet formed by laminating the aforementioned film on a porous supporting member which is pervious to printing ink and incapable of being substantially degenerated during the course of perforation of the film.
2. Description of the Prior Art
Heretofore, as means of preparing a heat-sensitive stencil sheet, a method has been known which comprises using as a heat source a visible irradiation and an infrared radiation generated by the flash irradiation method, causing the heat radiation to be absorbed in an original having letters, figures, and other patterns displayed thereon with a heat radiation absorbing substance, and allowing the absorbed heat to be transferred to an overlying film held in contact with the display part thereby melting to perforate the display substance to complete a perforated stencil. Besides, it has also been known to the art that a porous supporting member of non-woven, woven, or otherwise formed fabric of fibers pervious to printing ink is used as bonded to the film to prevent the letters or figures formed thereon from being erased accidentally during the course of perforation or printing.
Then, the method which prepares a stencil sheet by applying the electric power in the form of pulse signals selectively on the elements falling within a prescribed position through contact of the film with the heating elements and enabling the heat consequently generated to perforate the film has been known also in the art.
As concerns the versions of the former method, the specifications of Japanese Patent Publication No. 7623/1966 discloses a method which uses a stencil sheet obtained by laminating a stretched heat-sensitive resin sheet such as, for example, a stretched polypropylene sheet having a thermal shrinkage factor (area shrinkage factor during actual use) in the range of 0.3 to 2% with a thin tissue and effects perforation of the laminate with infrared radiation; the specification of Japanese Patent Publication No. 23713/1968 discloses a method which prepares a stencil sheet by similarly using a film manufactured by heat treating a stretched film of vinylidene chloride type resin so as to adjust the area thermal shrinkage factor of the film during the course of its actual use in the range of 0.5 to 3%; the specification of Japanese Patent Publication No. 10860/1974 discloses a method which prepares a stencil sheet by similarly using a film of an ethylene-vinyl acetate copolymer 10 to 70 .mu.m in thickness; the specification of Japanese Patent Application Laid-open No. 2513/1976 discloses a method which prepares a stencil sheet by similarly using a film of polyethylene terephthalate 4 to 20 .mu.m in thickness heat treated so as to have a density in the range of 1.375 to 1.385 (g/cm.sup.3), i.e. a degree of crystallinity in the range of 32 to 39%; and the specification of Japanese Patent Application Laid-open No. 85996/1985 discloses a method which prepares a stencil sheet by using a polyethylene terephthalate film having a thickness of 2 to 3.5 .mu.m and a longitudinal/lateral shrinkage factor of 2.5/1.9 (%) at 150.degree. C., for example.
As the versions of the latter method, the specification of Japanese Patent Application Laid-open No. 49519/1978 and that of Japanese Patent Application Laid-open No. 33117/1979 disclose a method which prepares a stencil sheet by perforating a commercially available film of crystallized polyethylene terephthalate through contact of the film with dots of heating elements and the specification of Japanese Patent Application Laid-open No. 49398/1985 discloses a method which prepares a stencil sheet by using a stretched film of polyethylene terephthalate not more than 4 .mu.m in thickness, on the condition that the melting point (m.p.) of the film 2 .mu.m in thickness should fall in the range of 255.degree. to 260.degree. C. to ensure satisfactory perforation.
The heat-sensitive stencil sheet which is prepared for printing with the perforation effected by the flash irradiation of an energy irradiation among other methods of perforation enumerated above is composed, as widely known in the art, by bonding a biaxially stretched thermoplastic resin film to a porous supporting member. At present, the film used in this stencil sheet is such that the effective perforation thereof is not effectively attained unless the flash irradiation of light is performed at a high energy level. In the case of the latter method, although the idea itself has been proposed, it has not been realized yet owing to various problems, including particularly the fact that no existing film is practicable because of lack of sensitivity high enough to cope with the thermal head of low energy level. In the circumstance, a study is being promoted with a view to overcoming the drawback by developing a thermal head capable of operating at heightened energy levels.
For the purpose of developing a film desirable for the formation of a stencil sheet, films formed of various biaxially stretched thermoplastic resins have been tested. All these films have various problems of their own which stand in the way of their practical adoption. The only stencil sheet grade films being accepted in the market are commercially available biaxially stretched polyethylene terephthalate film of high crystallinity having a thickness of 2 to 3 .mu.m and enjoying both dimensional stability and thermal resistance and biaxially stretched films of vinylidene chloride type copolymers 7 to 10 .mu.m in thickness. Even these films have various problems of their own.
The method which produces a printed copy by placing the perforated stencil sheet prepared as described above on a printing sheet of paper and applying stencil ink or screen printing ink on the stencil sheet thereby forcing the applied ink through the perforated letters or figures onto the underlying printing paper has been known to the art.
The conventional highly crystallized polyethylene terephthalate film used in the commercially available stencil sheets, because of its desirable workability (high modulus of elasticity enough to facilitate handling) and high dimensional stability, has found utility in the stencil sheet to be used in the automatic printing machine as a plate-making system relying for perforation on the flash irradiation method. The stencil sheet grade film disclosed in the specification of Japanese Patent Application Laid-open No. 48398/1985 and the specification of Japanese Patent Application Laid-open No. 85996/1985 are examples. These inventions are characterized by using films which have high degrees of crystallinity (such as, for example, at least 40% as determined by the density method). On the other hand, since these films have high crystal melting points, they cannot be easily used unless they have their thickness decreased to below 3 .mu.m for improving the perforation property, if only slightly. These known films have such main components which start shrinking at high temperatures on the order of 170.degree. C., for example. Owing to this shrinkage coupled with various other properties, the energy required for the perforation in such films is at a high level. The plate making by virtue of thermal energy necessitates use of an expensive xenon flash lamp of a large energy output. The films are mainly used in the region of such high energy. Moreover, the films to be used as stencil sheets must have a thickness as small as 2 .mu.m, for example, for the purpose of gaining in sensitivity as much as possible.
No further improvement of sensitivity can be expected of the films even when they lose their thickness appreciably any more. Their existing thickness is already at the limit. In this respect, as demonstrated afterward in comparative experiments, there are cases that a further decrease of thickness results conversely in degradation of sensitivity. This phenomenon is possibly ascribed to complicated factors which are involved during the perforation effected by the flash irradiation for an extremely brief period. It may be logically explained by a postulate that since the film is too thin for the heat to be stored sufficiently within the film, the heat imparted thereto is radiated instantaneously and the time required for retaining stress necessary for perforation is insufficient or by a postulate that the absolute value of the stress required for the perforation of the film as a whole dwindles. Besides, various problems such as the lack of efficiency of the manufacture of film, the possibility of the film sustaining ruptures at various steps of the production process, the lack of nerve in the film, the serious effects of static electricity generated, the occurrence of wrinkles, the inconvenience encountered during the work of lamination, and the loss of printing durability have suddenly come to attract keen attention. Inevitably, the existing films are expensive and unsatisfactory.
The biaxially stretched film of vinylidene chloride type copolymer which is generally used in the application in question has a slightly low perforation energy level as compared with the aforementioned polyethylene terephthalate film where the perforation is effected by the flash irradiation method and can be perforated with a flash lamp of a small energy output which fails to provide any ample perforation for the aforementioned polyethylene terephthalate film. Thus, perforation is effected at present by a method and apparatus which both prove simple and inexpensive.
This film, however, suffers from a disadvantage that when the perforation is effected by the flash irradiation method, and that by the use of a xenon lamp of high energy level, the resolving power thereof is degraded, i.e. the dots and lines of the perforated letters or images tend to be widened. Further, during the flash irradiation, such dots and lines tend to spread out by picking up dust, dirt, and surface irregularities of the original or they are fused to the original and, during the subsequent separation of the film from the original, the fused part tends to inflict a serious fracture on the perforated part of the film, damaging the film as a whole. There is another problem that the plasticizer contained in the film is decomposed with evolution of a corrosive gas at the elevated temperature.
Moreover, the film is deficient in dimensional stability and workability (at various steps of production process, including the formation of film, lamination of film on a supporting member, and perforation and printing performed on the stencil sheet). The film as a whole is also deficient in resolving power and printing durability. The film, therefore, finds utility barely in simplified printing machines for which the resolving power of lower degree suffices than the aforementioned automatic printing machine, especially in applications which have no use for prints of high quality. For example, the specification of Japanese Patent Application Laid-open No. 82921/1973 discloses a method which involves use of a vinylidene chloride type copolymer film amply heat treated so as to control the area thermal shrinkage factor in the working temperature zone in the range of 0.5 to 3.0%.
For the reason arising from the convenience of process, the aforementioned vinylidene chloride type film cannot be easily stretched to a thin thickness (high susceptibility to puncture and rupture and deficiency in strength and nerve (modulus)). Moreover, the physical properties, especially the stretchability, of the stretched film are liable on aging to be affected by the phenomenon of crystallization or by the action of the plasticizer, for example. As a natural consequence, the perforation property of the film is liable to vary. It is deficient in dimensional stability and liable to shrink. The film wound in a roll tends to shrink and, on being unwound from the roll and spread out, tends to sag and gather wrinkles. When this film is laminated with a supporting member with an adhesive and then dried, the resulting laminate shrinks heavily. To preclude the drawback, therefore, the film must be given a heat treatment to either mitigate or stabilize the orientation for the promotion of dimensional stability. This measure heavily affects the perforation sensitivity and must be carried out at a sacrifice of important properties.
The film has very weak nerve (modulus of elasticity) falling on the order of about 30 kg/mm.sup.2, a value notably low as compared with 400 to 600 kg/mm.sup.2 recorded for the commercially available polyethylene terephthalate, and further suffers from poor workability. The drawback, coupled with the disadvantages mentioned above, makes it hardly conceivable to use this film in a thickness somewhere around 2 to 3 .mu.m.
Since the existing films mentioned above have various problems, the appearance of a special film which is free from these problems, possesses a highly desirable performance warranting wide perforating conditions, high sensitivity, and high resolving power, and enjoys balanced properties is longed for.
This invention, in another aspect, relates to a method which effects perforation of a stencil sheet by the use of a thermal printer or thermal head used in the thermal printers for word processors, terminal devices, printers or facsimiles, i.e. the printers developed to keep pace with the rapid growth of electronic devices. The films used in this field are, therefore, expected to offer advanced properties including sensitivity of perforation and resolving power. The truth is that none of the films developed to date has satisfied the expectation. The printing resorting to the method of perforation under discussion has found no acceptance for the reasons for which the films are responsible. One particularly important requirement is that the perforation should be attained accurately and quickly with the heat of low energy. The films so far introduced invariably have much to be desired and are urging the necessity for further study. The thermal head to be used effectively for the perforation of the stencil sheet calls for a heat source of still lower energy level than any of the methods mentioned above.
Now, the perforation of the stencil sheet by the thermal head element will be described below. The thermal head of the existing principle has been adapted for the system which effects the printing by applying a wax of low melting point (such as 60.degree. C.) containing a dye (black or some other color) as an image developing medium to the film, melting the applied developing medium with the heat transferred from the head through the film, and transferring the molten developing medium to the copying paper (as adopted for the word processor, the facsimile, and printers) or for the system which effects the printing by heating the prescribed parts of a paper coated with a dye capable of reacting and producing a color on exposure to heat and causing the dye in the affected parts to form an image (as adopted for the facsimile, for example). In this field, the thermal head has been finding a rapidly growing market in recent years. In the applications mentioned above, the heat generating elements used in the thermal head are required to produce printed letters formed of dots of gradually decreasing size for the purpose of improving the quality of prints. A technically important point in the system is to make the printed letters clear with fine dots. A host of producers are competing among many manufacturers for early development of heat generating elements which satisfy the requirement.
Because of the great efforts devoted to improving the quality of the thermal head and to miniaturizing the elements, these heat generating elements are inevitably expensive. For the sake of service life, the voltage and amperage used on the elements during the course of printing are required to be decreased, the operating time (such as, for example, 0.2 to 4 msec/1 pulse) and the energy to be reduced, and the operating speed is required to be increased. Thus, the elevation of the printing speed constitutes one important requirement.
Since the thermal head is moving toward improvement of quality as described above, effective printing at a low energy level, prevention of the thermal head from deposition of refuse from decomposition or fusion, and prevention of the occurrence of corrosive gases and decaying matter are important requirements in view of elongation of the service life (generally accepted as 10.sup.7 -10.sup.8 pulse) of the thermal head.
This invention, in its another desirable aspect, relates to the stencil sheet and film without a supporting member (plane film) which are to be perforated by the heat perforation method using the aforementioned thermal head. When the aforementioned commercially available thermal head is used, the effective perforation required for the purpose of printing can hardly be attained on such stencil sheets as obtained by laminating the conventional films, i.e. crystallized polyethylene terephthalate film about 2 .mu.m in thickness and vinylidene chloride type copolymer film about 7 .mu.m in thickness, on supporting members (thin non-woven or woven fabric). Thus, it is entirely impossible to make any satisfactory printing by using the commercially available thermal head. In the circumstance, the thermal head necessitates modifications tending to increase the energy consumption by the heat generating elements, increase the magnitude of pressure exerted during the course of perforation, and decrease the printing speed, quite contrary to the requirements enumerated above. The efforts made for these modifications are far from those devoted to perfection of a thermal head which produces an image of fine picture elements, ensures an increased printing speed, and warrants enhanced durability.
As regards other known techniques, the specification of Japanese Patent Application Laid-open No. 48398/1985 has a disclosure to the effect that a polyester film not more than 4 .mu.m in thickness is used and this film, to be perforated satisfactorily, is required to be nothing other than a polyethylene terephthalate film 2 .mu.m in thickness (melting point 255.degree. to 260.degree. C.). In the specification of Japanese Patent Application Laid-open No. 48354/1985, the perforation of a stencil sheet using a polyethylene terephthalate film 2 .mu.m in thickness is mentioned. These inventions invariably reside in utility of the aforementioned commercially available highly crystallized polyester films which fall short of the level of perfection. Thus, various efforts are still being continued for development of films of improved quality.
For printing by the method using the heat-sensitive stencil sheet to nature into a new system satisfying a large market, successful development of an especially satisfactory stencil sheet, particularly a stretched film fulfilling the specific requirements described above, is an indispensable requirement.
When the existing commercially available heat-sensitive stencil sheet is perforated with the thermal head of a standard thermal-transfer type small desk-top word processor (for example, Casio-Word HW-120, produced by Casio Computor Co., Ltd.; a device furnished with a printing matrix of 16 dots.times.16 dots and designed to operate at a printing speed of 10 letters/second) (with the thermal-transfer tape cassette removed), no sufficient perforation is attained on the stencil sheet using the aforementioned polyethylene terephthalate film 2 .mu.m in thickness and the supporting member even at the highest possible printing energy level. The area of the perforated portion is about 15 to 20% of the part normally expected. The prints obtained by using the prepared stencil sheet are obscured so heavily as to defy deciphering. In the method of plate making by the flash irradiation, the stencil sheet using the vinylidene chloride type copolymer film about 7 .mu.m in thickness enjoys far better perforation property at a low energy level than the aforementioned polyethylene terephthalate film about 2 .mu.m in thickness. When this stencil sheet is perforated by the aforementioned thermal head, it exhibits far poorer perforation property than the stencil sheet using the aforementioned commercially available polyethylene terephthalate film about 2 .mu.m in thickness. The area of the perforated part is only about 2% of the area normally expected. The prints obtained by using the prepared stencil sheet are completely undiscernible. The reason for this phenomenon is not clear. This phenomenon, however, may be presumed that complicated film properties manifest their effects or the increase of film thickness acceleratedly degrades the efficiency of perforation. The word processor mentioned above is furnished with a serial thermal head intended for thermal transfer type and is operated with rather moderate heat energy and pressure. The transfer waxy ink for use with the thermal head is coated on the aforementioned commercially available crystallized polyester tape 3 to 3.5 .mu.m in thickness. The energy used in the thermal head, therefore, is controlled so that the pressure exerted for the perforation in this tape will not cause breakage of the tape. With the word processor of a higher grade offered by Casio and furnished with a printing matrix of 24 dots.times.24 dots and operated at a printing speed of 20 letters/sec., the stencil sheets using the two aforementioned commercially available films cannot be perforated at all. Since the word processors are moving toward higher operating speed and finer dot elements, the desirability of developing a new film of high performance capable of keeping pace with the rapid growth of the word processors has been expected.